Internal combustion engines, especially internal combustion engines used in motor vehicles, are nowadays generally fitted with a fuel injection system, e.g., a direct injection system. In this case, a control unit of the internal combustion engine determines, in particular, the start and end time of the fuel injection in such a way that the fuel quantity injected corresponds to a setpoint fuel quantity, which is determined, in particular, from the respective operating conditions. To ensure that the fuel quantity envisaged under the relevant conditions is fed to all the cylinders during operation and that as far as possible the same fuel quantity is fed to all the cylinders, the injection valves used in this context are specified with tight tolerances for the flow rate. Nevertheless, deviations from the setpoint fuel quantity can occur, e.g., due to deposits in the injection nozzles over the life of the engine. Such deviations can cause an exhaust gas composition which is unfavorable for exhaust gas aftertreatment, for example, can lead to unequal torque from the individual cylinders and hence to torsional vibrations in the drive train and can result in an increase in fuel consumption. For this reason, methods have been developed to determine the fuel quantity actually injected into a cylinder of the internal combustion engine and to adapt the injection parameters accordingly on this basis, for example, thus allowing the envisaged fuel quantity to be fed to the cylinder during the operation of the internal combustion engine.
DE 10 2010 036 485 B3 discloses a method for controlling an internal combustion engine having a fuel injection system, wherein an injection duration of an injection valve of the internal combustion engine is modified, an exhaust gas composition of the internal combustion engine is measured, a deviation in a flow through the injection valve from a nominal value is determined from the exhaust gas composition, and the injection valve is activated with an injection duration adapted to the compensation of the deviation determined.
According to EP 1 336 745 A2, the intake mass flow and air ratio in the exhaust gas are measured in a method for controlling the injection in a motor-vehicle internal combustion engine in order to estimate the fuel quantity which has actually been injected into the engine. By means of a closed control loop, it is ensured that the estimated fuel quantity is substantially equal to a setpoint fuel quantity calculated to meet the requirements of a user of the motor vehicle. Here, the difference between the setpoint fuel quantity and the estimated fuel quantity is used to calculate a correction factor with which the setpoint fuel quantity is corrected.
GB 2 475 521 A discloses a method for determining the actually injected fuel quantity in an internal combustion engine, wherein a setpoint test fuel quantity is injected into a cylinder in an operating state without fuel supply, the air quantity which flows into the cylinder during the test injection is determined by means of an air mass flow sensor, the air ratio in the exhaust gas is determined by means of a lambda probe, and the fuel quantity injected in the test injection is calculated on the basis of a function of the air quantity which has flowed in the cylinder and on the basis of the air ratio. The fuel quantity injected in the test injection can be compared with a setpoint fuel quantity in order to correct injection, e.g. by corresponding adjustment of the activation duration of the injection valve.
However, the inventors herein have recognized an issue with the above approaches. In one example, the approach described above, wherein the fuel quantity injected in the test injection is calculated on the basis of a function of the air quantity which has flowed in the cylinder and on the basis of the air ratio, may lead to inaccurate fuel injection corrections owing to the difficulty of measuring the air quantity which flows into the cylinder during the test injection and/or due to a deviation in the air quantity measurement due to, for example, humidity. For example, the air mass flow sensor may determine the air mass flowing to each cylinder, and it may be difficult to estimate how much air actually flowed to the cylinder receiving the test injection.
Thus, the inventors herein provide an approach to at least partly address the above issues. In one example, a method includes, in a deceleration phase, forming a closed exhaust gas recirculation circuit, determining a mass (mcirc) of an air quantity enclosed in the closed exhaust gas recirculation circuit, detecting a first oxygen content of the air quantity, carrying out a test injection with an injection valve, detecting a second oxygen content of the air quantity, and determining a fuel mass (mfuel) injected in the test injection from the mass (mcirc) of the air quantity and the first and the second oxygen contents.
In this way, the mass of air enclosed in the closed exhaust gas recirculation (EGR) circuit may be determined, which may be more accurate than measuring the air mass flowing to a single cylinder. The amount of oxygen consumed by the injected fuel in the test injection may be determined by the difference between the first and second oxygen contents, and the amount of consumed oxygen used to determine the actual amount of fuel delivered during the test injection. In one example, if the amount of fuel injected during the test injection is different than expected, future fuel injection amounts may be adjusted accordingly, thus increasing the accuracy of the delivered fuel injection amounts.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.